A biogas may be produced through the anaerobic digestion of a material containing biomass. The biogas is typically comprised of 50-75% methane and 25-50% carbon dioxide. Other gases, such as nitrogen, hydrogen, hydrogen sulfide or oxygen may be also present but collectively are unlikely to account for more than 10% of the biogas. Of these other gases, nitrogen is likely to be the largest component. The biogas can be burned directly with oxygen, for example, and so is usable as a fuel. The methane within the biogas can also be concentrated to provide a replacement for natural gas.
Biogas can be produced in an anaerobic digester. The digestion process involves microorganisms, primarily bacteria, which break down or convert the input materials to produce the biogas and an effluent. The process involves a series of bacteria types and processes, primarily hydrolysis, acidogenesis, acetogenesis and methanogenesis.
Anaerobic digesters were originally designed primarily for use with cattle manure and sludges. The sludge may be, for example, waste sludge from a municipal wastewater treatment plant. Municipal wastewater, or sewage, is typically treated using an activated sludge process with primary clarification, a biological process train, and secondary clarification. Waste activated sludge, optionally thickened, and primary sludge may be fed to an anaerobic digester at a dry solids (DS) concentration of up to about 4%. The digester typically operates at 2 to 2.5% solids concentration in the digester with a 20 to 25 day hydraulic retention time (HRT).